An electrophotographic printer utilizes a developer mixture to form images on media. The developer is made up of two parts, magnetic carrier, and toner (dry ink). In order to maintain a constant dry ink concentration, the voltage level of the dry ink monitor must be maintained as close to a target voltage level as possible. Machine software adjusts the dry ink concentration to increase or decrease the amount of dry ink in the developer mixture to reach the target voltage level. For instance, the dry ink concentration can be varied by a replenisher that adds dry ink to the developer station, thereby decreasing the voltage response of the dry ink monitor. However, the thermal drift coefficient of each individual sensor varies from sensor to sensor since the sensors are inherently sensitive to temperature changes and the dry ink concentration can vary considerably during operation depending on the temperature. Also, the dry ink concentration sensor typically may exhibit positive or negative thermal drift, which may change over the life of the sensor.
The prior art includes systems that apply corrections to the toner magnetic sensors based upon temperature, burn-in, and toner-age. For example, U.S. Pat. No. 6,175,698 identifies that temperature, burn-in, and toner-age of the toner particles in each developer structure can affect the amount of toner required to develop the latent image. The sensor of the '698 patent is used to obtain the toner concentration readings, but cannot directly measure the actual toner concentration. Thus, the readings are adjusted by combining the target toner concentration to provide a error signal that is input to control feedback dispensing of the toner. The feedback dispenser of the '698 patent processes the error signal and commands the developing station to request that a certain toner mass per unit time be dispensed to compensate or correct for variations in temperature, burn-in, or toner-age to attempt to maintain the proper toner concentration.
However, these and other known sensors are susceptible not only to temperature fluctuations while in service in the machine, but are also susceptible to thermal drift of the sensors themselves.
Thus, a method and apparatus is needed that can compensate for the thermal drift of the sensors.